Can't push boulders through iron bars; traps can't roll such through either;
likewise for objects thrown by monsters.
Thrown objects susceptible to breaking might do so when they hit iron bars.
Assorted monsters can pass through iron bars; ditto for polymorphed character.
Attempting to dig iron bars will wake nearby monsters instead of yielding
"you swing your pick-axe through thin air".
Autodig won't accept iron bars as candidate location.
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